The invention relates to a double-walled pipe assembly for transmitting a fluid and, more particularly, the invention relates to a double-walled pipe assembly with leak detection capabilities for safely conveying environmentally hazardous fluids.
While the invention is particularly applicable to double-walled pipe assemblies used in the petroleum, chemical, and natural gas industries, it should be appreciated that the pipe assembly can also be utilized in connection with all types of pipe installations in which leakage of an environmentally hazardous fluid may occur. The leakage of environmentally hazardous fluids in small amounts over long periods of time can produce a serious environmental hazard which is difficult to detect and difficult and expensive to clean up once it has ultimately been detected.
Conventional underground fluid piping systems, such as those utilized in service station environments, are typically made of steel, fiberglass, or plastic. Such systems generally include rigid straight lengths of pipe connected with T-fittings, elbows, connector fittings, union fittings, and the like. The assembly of these multiple components creates a piping system with many joints which are often prone to leakage. In addition, rigid piping systems are adversely affected by ground movement occurring during the life of the system which may cause failures of the pipes and connections.
Due to ever stricter environmental regulations propounded by the government concerning underground piping systems carrying environmentally hazardous fluids, piping manufacturers have developed a variety of secondary containment systems which are designed to prevent any fluid that may leak from a fluid transmitting pipe from escaping into the environment. One such approach for a secondary containment system involves lining a piping trench with a flexible membrane liner or semi-rigid trough to provide containment of a leaking fluid. This technique provides successful leak containment but does not provide a method of detecting leaks or determining a location of the leak in the piping line.
Another approach to the secondary containment problem utilizes a rigid, larger diameter pipe surrounding and enclosing a conventional rigid fluid supply pipe. With such an arrangement, the pipe fittings of the larger diameter containment pipe are an area where leakage of fuel to the environment may occur.
An additional type of secondary containment piping system utilizes spaced access chambers interconnected by a secondary containment pipe which provides a sealed housing around a flexible fluid supply pipe. The ends of the flexible fluid supply pipe are disposed within the access chambers and the flexibility of the inner supply pipe allows the inner pipe to be removed and replaced through the access chambers if a leak occurs. However, in practice, this removable inner supply pipe is difficult to remove because the pipe is resistant to the type of bending necessary to pull it out of the secondary containment pipe. This removable inner pipe system may include sensors for leak detection. However, this system does not allow identification of the location of a leak within the containment system.
A recent development in secondary containment of hazardous fluids employs a flexible co-axial piping system in which an outer pipe surrounds a fluid carrying inner pipe and the outer pipe contains any fluid which leaks from the inner pipe. These existing coaxial fuel delivery systems utilize a non-reinforced outer pipe which is capable of holding a maximum fluid pressure of about 5 psi (34.5 kPa) above atmospheric. Typical fuel pressures are much higher than this, for example fuel pressures from 30 to 60 psi (207 to 404 kPa) are common. Thus, a sudden failure of the inner pipe and the attendant fluid pressure on the outer pipe could cause a rupture of the outer containment pipe resulting in leakage of fuel to the environment. Thus, it would be desirable to provide a containment system which will contain both slow leaks and catastrophic failures of the inner pipe without damage to the environment.
Accordingly, it would be desirable to provide a secondary containment system for transmitting environmentally hazardous fluids which (1) prevents catastrophic failures, (2) provides leak detection, and (3) allows determination of the particular location of the leak in the piping system.